During alias CTAD, we're accidentally ignoring the aggregate deduction
candidate of the underlying template because it's added to the candidate
set separately via maybe_aggr_guide (which doesn't yet handle alias
templates) rather than via deduction_guides_for (which does). This
patch makes maybe_aggr_guide handle alias templates in a manner similar
to deduction_guides_for.
Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
trunk?
PR c++/98832
gcc/cp/ChangeLog:
* pt.c (maybe_aggr_guide): Handle an alias template.
gcc/testsuite/ChangeLog:
* g++.dg/cpp2a/class-deduction-alias9.C: New test.
---
gcc/cp/pt.c | 11 +++++++++++
gcc/testsuite/g++.dg/cpp2a/class-deduction-alias9.C | 6 ++++++
2 files changed, 17 insertions(+)
create mode 100644 gcc/testsuite/g++.dg/cpp2a/class-deduction-alias9.C
diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index aeb1e8a6f97..db15e4714d5 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -28880,6 +28880,8 @@ is_spec_or_derived (tree etype, tree tmpl)
return !err;
}
+static tree alias_ctad_tweaks (tree, tree);
+
/* Return a C++20 aggregate deduction candidate for TYPE initialized from
INIT. */
@@ -28892,6 +28894,15 @@ maybe_aggr_guide (tree tmpl, tree init,
vec<tree,va_gc> *args)
if (init == NULL_TREE)
return NULL_TREE;
+ if (DECL_ALIAS_TEMPLATE_P (tmpl))
+ {
+ tree under = DECL_ORIGINAL_TYPE (DECL_TEMPLATE_RESULT (tmpl));
+ tree tinfo = get_template_info (under);
+ if (tree guide = maybe_aggr_guide (TI_TEMPLATE (tinfo), init, args))
+ return alias_ctad_tweaks (tmpl, guide);
+ return NULL_TREE;
+ }
+
/* We might be creating a guide for a class member template, e.g.,
template<typename U> struct A {
diff --git a/gcc/testsuite/g++.dg/cpp2a/class-deduction-alias9.C
b/gcc/testsuite/g++.dg/cpp2a/class-deduction-alias9.C
new file mode 100644
index 00000000000..0aaf203639a
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/class-deduction-alias9.C
@@ -0,0 +1,6 @@
+// PR c++/98832
+// { dg-do compile { target c++20 } }
+
+template<class T, class U> struct X{ U u; };
+template<class T> using Y = X<int, T>;
+Y y{0};
--
2.32.0.93.g670b81a890
